Remember what "intumescent" means - it gets bigger when it gets hot. These materials are used as firestops where, say, a 3-1/2" plumbing stack might burn away leaving a hole for fire to rise through a building. The firestop here is a few wraps of intumescent tape held in place so that as the pipe burns away, the material expands to fill the hole. Now fast forward to your flue pipe - imagine what happens to that caulk the first time the furnace fires up!

Silicone is fine, a stainless bolt (cheap when you buy them by the 100's), a purpose made plug, or a piece of 181 tape are all OK, as well as is doing nothing.

I use metal plugs that snap in. I buy them from my local electronics store. They use them to seal holes in electronics cabinets. They are available on the web as well Use the link below and see if they are what you want. You can always coat them with high temp rtv if you want a better seal

I use 1/4" hole plugs. Try checking at www.sidharvey.com They have them for a good price. I do not have they're part # handy. But they are also available at Grainger www.grainger.com The part # at Grainger is (4HFR1).

For drilling I use a 1/4" drill bit, and sometimes have to ream the hole a little for them to fit correctly. They should snap in tightly, and can be removed/replaced when testing in the future. You may be able to order a set with the appropriate bit included.

I've seen tape many years old covering holes. It is not likely to go anywhere, and if it does, is it not going to hurt anyone or cost any energy?

You are unlikely to change how this is performed to a meaningful amount, and if you did have you wasted more resources than you've saved? Is the hole going to be the difference between drafting and not drafting. I think covering it is mostly of cosmetic importance to prevent homeowners from worrying about a non-existent problem.

I'm with Ed, picking at nits here. I may be wrong 5 times a day, and this may be one of them, but to me this is an added cost with no value. From an economic perspective all costs we assume are eventually borne by the client, and this is not something I want to put my hand in their pocket for.

In the give and take of life not lived in a vacuum, if you spend additional time on this where are you taking the time from? If we have to buy solid gold plugs that take 15 additional minutes to install, who does that help? If you spend money on this where are you taking the money from?

Possibly from things of greater value to the homeowner?

I see Joe Miuccio replied - Joe, what is the reason you use these plugs - Health, Safety, to satisfy your OCD, or to avoid calls from OCD homeowners?

I understand we're you are coming from on this one. But, the difference in material cost and labor is basically negligible. Some times you just have to eat some costs to further not just your own business but the industry as a whole.

I take this same approach to 95% of the jobs I perform for homeowners. Did I need to tape those last few seams? Probably not, but it just means that I cared all that much more to do it anyway. It starts with the small stuff...

Patrick, I like Bruce's response, tells me my intuition; "I think covering it is mostly of cosmetic importance to prevent homeowners from worrying about a non-existent problem." is probably dead on.

Any cost you bear once in a while my be a cost to you, but any cost you bear as part of your business process is paid by the customer. Unless you have a big trust fund, that's simple economics. At some point every negligible cost and 2 minute investment, if providing no true added value to the customer, adds up to significant added cost TO THEM. Spending time and effort to get some deburred plug fitting pretty, which saves neither health nor energy, is time and effort I would rather spend educating my clients about building science.

And that does NOT compare to taping those last seams!! THAT MOST DEFINITELY adds value on the energy side, and if they are return seams, maybe on the health side as well!!

As someone who seems obsessively detail oriented, please don't even go down the path of comparing "should I tape those last seams, or plug the pressure testing holes..." because one is very important, the other is not.

Look, there will always be more you would like to do. I'd really like to take ESP, filter restriction, and coil restriction measurements on homes. But homeowners are already asking me if they should make up the guest room. Really, you can't move in on people, they get tired of you (or at least me).